Thermoplastic polyesters such as polybutylene terephthalate have been and are attempted to be used in some of machine parts, parts of electric appliances and auto parts as engineering plastics, making use of their excellent properties such as mechanical strength, rigidity, heat resistance, chemical resistance and oil resistance. However, since thermoplastic polyesters such as polybutylene terephthalate are inferior to polycarbonates in impact resistance in spite of the above excellent properties, proposals have been made to improve their impact resistance.
As exemplary means of improving the impact resistance of thermoplastic polyesters, there are proposed a method in which a polyester is mixed with ethylene.propylene rubber, polyisobutene or polybutene (JP-B-46-5225), a method in which a thermoplastic polyester is melt mixed with a modified ethylene polymer having a crystallinity of 75% or less obtained by graft polymerizing .alpha.,.beta.-unsaturated carboxylic acid or a derivative thereof (JP-B-57-54058, JP-B-57-59261), and a composition comprising a thermoplastic polyester and a specific ethylene..alpha.-olefin random copolymer having a polar group obtained by graft-polymerizing a derivative of .alpha.,.beta.-unsaturated carboxylic acid or unsaturated epoxide (JP-B-59-28223). However, the former method is insufficient in impact resistance-improving effect because the polyester has poor compatibility with the ethylene.propylene rubber. Although the latter two methods improve impact strength, its improvement is still insufficient, and in order to obtain sufficient impact strength, it is necessary to increase the amount of a flexible resin composition to be melt kneaded with a polyester, by which rigidity inherent in a thermoplastic polyester is lost.
Further, a composition obtained by melt mixing a thermoplastic polyester with a modified ethylene polymer prepared by graft polymerizing .alpha.,.beta.-unsaturated carboxylic acid or a derivative thereof is inferior in toughness against tensile stress to a polyester which does not contain a modified ethylene polymer.
A composition prepared by adding a copolymer of .alpha.-olefin and a glycidyl ester of .alpha.,.beta.-unsaturated acid and an ethylene copolymer to a polyester and mixing these (JP-A-58-17148) and a composition prepared by adding an ethylene..alpha.-olefin copolymer obtained by graft polymerizing an unsaturated carboxylic acid or an anhydride thereof and a polyepoxy compound to a polyester and mixing these (JP-A-60-28446) are disclosed as compositions having excellent impact resistance at low temperatures. However, although the impact resistance at low temperatures is significantly improved in these compositions, it is still insufficient in some of their applications. When the amount of a flexible resin composition to be melt kneaded with a polyester is increased to obtain sufficient impact strength, rigidity inherent in the thermoplastic polyester is lost like the above description.